That seems easy to optimize out, especially given rust's mutability guarantees. If I have a u8 and I index into an array in a loop the compiler should be free to cast it to a usize above that loop, right?

The benefit being you can store much smaller structures, plus in the case of memory unsafety you reduce attacker control.

The vast majority of strings I personally use are certainly well under 2^32 bytes, and most are probably under 2^8.